An ultrasonic diagnostic apparatus is known as a subject information acquisition apparatus. The ultrasonic diagnostic apparatus noninvasively images information inside the subject, so that the ultrasonic diagnostic apparatus is widely used in the medical field.
In the ultrasonic diagnostic apparatus, an ultrasonic cross-sectional image in which a structure in a biological body is imaged from reflection echo generated by differences of reflectivity of tissues and a Doppler ultrasound image in which a blood flow velocity or the like is imaged by using an ultrasonic Doppler effect generated by a blood flow are conventionally used.
Further, recently, hardness of tissue is measured by an ultrasonic wave and the hardness is used to diagnose the tissue. This is because the hardness of tissue deeply related to a pathological state. For example, it is known that a sclerosing cancer such as breast cancer and thyroid cancer is harder than a normal tissue and a benign tumor.
In recent years, it is reported that viscosity is different between cancer and a benign tumor, so that not only tissue hardness measurement, but also viscoelastic characteristics evaluation including viscosity characteristics evaluation is required for tissue diagnosis. Therefore, it is required to measure distribution of viscoelastic characteristics in a biological body by using ultrasonic wave, form the distribution into an image, combine the image with a conventional image of tissue structure, and use the combined images to diagnose cancer or the like in a tissue.
As an ultrasonic diagnostic apparatus for calculating the viscosity of the subject, PTL 1 discloses an apparatus that uses an ultrasonic probe in which a pressure sensor is provided on a surface of a transducer transmitting and receiving an ultrasonic wave, measures strain distribution by ultrasonic wave, measures pressure distribution applied to the subject by the pressure sensor, and calculates values of elasticity and viscosity from the strain distribution and the pressure distribution.
As a method for measuring pressure, PTL 2 discloses an ultrasonic diagnostic apparatus in which a pressure measuring deformation portion whose elastic modulus is known is sandwiched between an ultrasonic probe and a biological tissue to measure pressure. The ultrasonic diagnostic apparatus measures deformation of the pressure measuring deformation portion by ultrasonic wave, calculates pressure (stress) applied to the pressure measuring deformation portion from a relationship between elastic modulus and strain, calculates elastic modulus distribution in the subject from the stress and strain distribution in the subject, and displays the elastic modulus distribution.
However, in the configuration described in PTL 1, the structure of the ultrasonic probe is complicated. Further, generally when strain distribution in a tissue is measured by using ultrasonic wave, applied pressure is very small, so that it is difficult for a normal pressure sensor to measure the pressure. Furthermore, if the pressure sensor is inserted immediately below the ultrasonic probe, efficiency and sensitivity of transmitting and receiving ultrasonic wave degrade, so that there is a problem that measurement of deep portion cannot be performed.
In PTL 2, although the configuration of the apparatus is simple, only the elasticity distribution in the subject is displayed and a method for measuring the viscosity distribution in the subject is not disclosed. Therefore, an apparatus which has a simple configuration and which can calculate viscosity of the subject is required.